Carbon fibers, generally made of acrylic fibers or pitch (by-products of petroleum oil, coal, coal tar, and the like) carbonized at high temperatures, are obtained by heating and carbonizing precursors of organic fibers, and defined as the fibers composed of carbon in the ratio of 90% or more according to Japanese Industrial Standards (JIS). Because the carbon fibers have lower dispersibility than other fibers and each fiber of the carbon fibers is entangled with each other weakly, they are scarcely used to form nonwoven fabrics as the only material, but are usually used as a composite material with resin fibers such as synthetic resin.
For example, Patent Document 1 discloses a method, wherein carbon fibers of 20 to 70% by weight and binder fibers of 30 to 80% by weight are mixed to form nonwoven fabrics, and then the nonwoven fabrics are burnt for removal of the binder fibers to obtain nonwoven fibers composed of the carbon fibers.
Another method is also known, which is to produce nonwoven fabrics by increasing tangles of carbon fibers with thermoplastic resin fibers, wherein staple-like short fibers of the carbon fibers and the thermoplastic resin fibers are blended to make a sheet, and the sheet is laminated, and then the carbon fibers and the thermoplastic resin fibers are entangled by a needle punch method (for example, see EXAMPLES of Patent Document 2).